One of the most important considerations in designing an optoelectronic system is the connection between circuit components such as integrated circuits or optoelectronic devices with purely optical components such as waveguides. Sometimes, connections are needed between a first component mounted upon a printed circuit board and a second component located on the same board or on another board. Occasionally, connection is desired between a component mounted upon a printed circuit board and a connector located at the edge of the same board.
One type of connector that is presently utilized to connect optoelectronic devices to optical fiber is called an optical fiber “pigtail” connector. A fiber pigtail is a length of optical fiber, generally having a portion of its buffer coating removed and often coated with a metallized coating. Such pigtail connectors or couplings can optically couple an optical fiber or lensed optical fiber with light-producing or light-detecting elements coupled to leads of the integrated circuit. The light-producing element, for example, can be a semiconductor or diode laser, or a light-detecting element could be a photodiode. These elements are typically coupled with and fixedly disposed and oriented to an end of the lensed optical fiber, which is located in such a manner as to reside within the housing often forming a hermetic seal therewith. The lensed or collimating fiber extends beyond the edge of the housing and into it carrying signals to and/or from the integrated circuit. Mounting the fiber pigtail to the housing is not a trivial procedure. For example, alignment of a laser diode mounted within the housing with a lensed end of the fiber pigtail must be such that maximum coupling of light exist from a laser to the fiber. Furthermore, the coupling must be robust maintaining optimum alignment and coupling over time and often through changes in temperature and humidity. This coupling should preferably be tolerant of being handled or even dropped.
One common method of forming a relatively secure fixed connection between an optoelectronic component such as a photodiode and an optical fiber within a same package is to coat a length about an end of an optical fiber pigtail with metal so that it can more easily be soldered into position to metal pads within a housing. For example, U.S. Pat. No. 6,146,025 discloses a package wherein an end of an optical fiber is stripped down to the cladding having a portion of the outer jacket removed and wherein the stripped clad portion is metallized before being soldered to provide a strong bond with the solder. Heretofore, it is customary to fabricate fiber pigtails that are to be soldered, by pre-stripping the outer jacket and metallizing the entire end, often including a portion of the jacket about the stripped end. This is shown more clearly in FIG. 1 wherein an optical fiber 20 has a stripped portion 22 coated in a thin layer of metal 24 to be soldered to solder pads 26 within a housing.
One problem with arrangement, is that the solder joints and the fiber can be damaged due to unwanted heating of the fiber from the laser light, leading to a thermally induced reliability problems. Absorption of high power cladding light by the fiber metal can cause the fiber metal to overheat in a region of the fiber in which there is a minimal thermally conductive path for heat dissipation; this typically occurs between the fiber mount and or the region about the opening in the package where the fiber is soldered and the region therebetween. The thermal resistance of a fiber is very high, and the thin metal layers normally used with metallized fiber have high thermal resistance as well. As a result the fiber metal may reach temperatures that can cause the metal to oxidize, or form undesirable alloys among the different materials within the coating. The metal may crack, interrupting the conductive thermal path and resulting in additional overheating. In the worst case the fiber may crack resulting in the failure of the device.
Furthermore, this absorption can also cause localized heating of solder used to bond the metallized fiber to an alignment structure or to a welding ferrule. Localized heating within the fiber and solder may result in fiber alignment shifts from plastic solder yielding because of local thermal stress variations, the redistribution of residual stresses, and creep deformation in the solder. An alignment shift will cause decoupling of light energy from the fiber core and an increase in cladding mode energy, resulting in a thermal runaway situation, with eventual total loss of coupled power.
It is an object of this invention, to provide a fiber pigtail for physically bonding to a laser package and optically coupling with a diode laser that lessens the problems described heretofore with conventional stripped and metallized fibers for soldering in high power laser packages.
In accordance with the invention, there is provided, an optical fiber pigtail assembly comprising a span of optical fiber, having an end for receiving light from a laser to be optically coupled therewith, wherein a first region of said optical fiber about th e end is metallized for soldering in a fixed position relative to said laser and wherein a second region of the optical fiber downstream and spaced from the first region is metallized for further fixedly mounting said fiber tail assembly relative to said laser, and wherein an other region of said optical fiber between said first region and said second region is absent metallization to minimize heating of said optical fiber when high intensity light propagates therethrough.
In accordance with another aspect of the invention, there is provided, a method of fabricating an optical fiber pigtail assembly comprising the steps of:
fixedly providing a laser diode within a housing in a predetermined orientation;
stripping a portion of an optical fiber to provide an optical fiber pigtail;
coating to metallize at least two separate regions of the optical pigtail so as to leave an uncoated region there between, the uncoated region being substantially larger than each of the coated regions;
grasping the pigtail end with tweezers having non-metallic grasping ends;
placing the optical fiber pigtail in the housing such that an end of the pigtail is oriented to couple light from the laser diode; and,
soldering the at least two metallized regions to solder pads within the housing.
In accordance with another aspect of the invention, there is further provided, An optical fiber pigtail assembly comprising:                a) a laser diode disposed within a housing; and,        b) an optical fiber pigtail at least partially disposed within an opening of said housing, said optical fiber pigtail having an end including a lens for coupling light there into from the laser diode, the optical fiber pigtail having a first region about the end that is metallized and soldered fixedly with respect to said housing, the optical fiber pigtail having a second metallized region spaced from the first region by an other region wherein the second region is soldered to the housing about the opening to provide a seal therewith, said other region being absent of metal to less en heating that may otherwise occur in the presence of a high intensity light from the laser passing therethrough.        